1. Field of the Invention
The invention relates to a pyrimidine derivative for use in the hardness stabilization of rubber and a method of improving the hardness stabilization of rubber by adding the pyrimidine derivative and associated accelerators to an unvulcanized rubber composition.
2. Discussion of the Prior Art
Vulcanizing rubber compositions by heating a sulfur-vulcanizable rubber composition with sulfur and/or a sulfur donor and a vulcanization accelerator has been known for many years. By this process vulcanizates having acceptable physical properties including tensile strength, resilience, and fatigue resistance can be obtained, but such vulcanizates tend not to have good aging properties. A typical aging phenomenon is hardening, which is explained below.
Uncured as well as cured rubbers are prone to aging effects. The unsaturated groups in diene rubbers, e.g. styrene-butadiene rubber (SBR) or a blend of SBR with natural rubber, butadiene rubber or with both, make it possible to cure with sulfur, but at the same time they exhibit a sensitivity toward oxygen, ozone, and other reactive substances causing changes such as hardening of the vulcanizate. Unaged diene rubbers contain free double bonds that remain sensitive to the above reactive substances even after vulcanization. Higher temperatures make these effects even more noticeable. Also, since unreacted double bonds are present in the rubber vulcanizate, there is the possibility of further reaction with sulfur causing hardening, i.e. additional crosslinking, of the vulcanizate.
The use of antioxidants will retard the oxygen-induced aging of the vulcanizate, but will not affect the increase in hardness due to sulfur-induced crosslinking.
L. H. Davis et al. in Rubber Chemistry and Technology, Vol. 60, 1987, 125-139, disclose the use of 2,2xe2x80x2-dithiobispyridine-N-oxide and the zinc salt of pyridine-2-thiol-N-oxide as a primary accelerator alone or in combination with a low amount of a benzothiazole-2-sulfenamide accelerator in the sulfur vulcanization of polyisoprene, e.g., natural, rubber compounds.
U.S. Pat. No. 3,574,213 discloses rubber vulcanization accelerators comprising pyrimidinylthio-phthalazines, particularly 1-(4,6-dimethyl-2-pyrimidinylthio)-phthalazine, that achieve reduction in scorch.
C. J. Rostek et al, in Rubber Chemistry and Technology, Vol. 69, 1996, 180-202, disclose the use of novel sulfur vulcanization accelerators based on mercapto-pyridine, -pyrazine, and -pyrimidine. This reference relates to polyisoprene rubbers, which do not harden.
U.S. Pat. No. 3,839,303 discloses the inhibition of premature vulcanization of natural or synthetic diene rubbers by including in the vulcanizable composition accelerating agents, such as thiazole accelerators and a compound comprising certain pyrimidinesulfenamides, such as N-cyclohexyl-4,6-dimethyl-2-pyrimidinesulfenamide. The compound of this reference is formulated so as to be effective in inhibiting premature vulcanization in the vulcanizable composition to which it is added.
U.S. Pat. No. 3,565,894 discloses heterocyclic esters of dithiocarbamic acids wherein the heterocycle contains 3 or 4 carbon atoms and two meta nitrogen atoms and is attached at the carbon between the nitrogen atom, which esters are useful for accelerating the vulcanization of rubber.
In one embodiment, the present invention comprises a vulcanizable composition comprising a sulfur vulcanizable rubber, a sulfur vulcanizing agent, an accelerating agent and a hardness stabilization agent comprising a pyrimidine derivative of the formula: 
Where X is S, O or NH, Y is N, SR3, or OR3, R1 through R4 are the same or different and selected from the group consisting of the substituents alkyl, cycloalkyl, alkylaryl, aryl and aralkyl, hydrogen, halogen, hydroxy, amino, substituted amino, and substituted carbonyl containing groups, R3 may be a radical derived from a carbon based heterocyclic group containing at least one of S or N, or both S and N, and R3 and R4 may be in the same constituent together with N to form various heterocycles, and R3 may also be a radical derived from heterocyclic groups, and the respective amounts of accelerating agent and hardness stabilization agent being effective to not substantially inhibit vulcanization and to stabilize the hardness property of said rubber upon vulcanization, the respective amounts of accelerating agent and hardness stabilization agent being effective to not substantially inhibit vulcanization and to stabilize the hardness properties of said rubber upon vulcanization, the amount of accelerating agent in said composition being greater than about 0.6 phr when said rubber is SBR rubber, or at least about 0.5 phr when said rubber is natural rubber, and the amount of hardness stabilization agent being at least about 0.5 phr.
In a second embodiment, the present invention comprises a method of improving the hardness stabilization of rubber which includes adding the above composition to an unvulcanized rubber composition followed by vulcanization of the rubber composition.
Other embodiments of the invention encompass specific pyrimidine derivatives, accelerators, details about relative amounts of reactants, and unvulcanized rubber compositions, all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.
According to the present invention, it has been found that by adding appropriate amounts of certain pyrimidine derivatives and accelerators to a vulcanizable rubber composition comprising natural rubber or other rubbers, vulcanizates having improved properties can be made, from which, e.g., pneumatic tires can be made. These combinations of accelerators and pyrimidine derivatives have the effect of stabilizing the hardness properties of the rubber vulcanizate, e.g., during the service life of a pneumatic tire, without inhibiting or slowing vulcanization, i.e. increasing xe2x80x9cscorchxe2x80x9d time, in the production of the tire. Thus, hardness stabilization is achieved without slowing of the vulcanization process, thereby avoiding loss in production efficiency.
In this application, the abbreviation xe2x80x9cphrxe2x80x9d means the number of parts by weight per 100 parts by weight of rubber. In the case of a rubber blend, it is based on 100 parts by weight of total rubber.
Either natural rubber (NR), styrene-butadiene rubber (SBR) or a blend of NR and SBR or NR and SBR with one or more other rubbers can be used in the invention process, it being understood that for purposes of this invention the term xe2x80x9crubberxe2x80x9d means an elastomer containing a hydrocarbon unit which is a polymer with some unsaturated chemical bonds. Typically, SBR, a blend of SBR with natural rubber (NR), a blend of SBR with polybutadiene rubber or butadiene rubber (BR), or a blend of SBR with NR and BR is used. The type of rubber or mixture of rubbers will have some affect on the precise amounts of accelerator and pyrimidine derivative appropriate to achieve hardness stabilization without inhibition of the vulcanization.
Typically, the amount of pyrimidine derivative hardness stabilizing agent employed in the process of the present invention will be from at least about 0.5 phr to preferably about 10.0 phr. and most preferably about 3.0 phr.
In the presence of the present invention sulfur and/or a sulfur vulcanizing agent is employed. The amount of sulfur to be compounded with the rubber usually is 0.1 to 10 phr, preferably in excess of about 1 phr. If a sulfur donor is used the amount thereof should be calculated in terms of the amount of sulfur.
Typical examples of sulfur donors that can be used in the process of the present invention include dithiodimorpholine, caprolactam disulfide, tetramethylthiuram disulfide, and dipentamethylenethiuram tetrasulfide. The reader is referred to W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989, in particular pages 231-233.
Particularly preferred pyrimidine derivatives for us in the composition and method of the present invention have the following chemical structural formulas:
PD1: 4,6-Dimethyl-2-pyrimidyl-N,N-dimethyldithiocarbamate. CAS registration number is [32595-22-5]
PD2: S-(4,6-Dimethyl-2-pyrimidyl)-N-n-butylthiocarbamate.
CAS registration number is [61887-72-7] Reference: H. Voigt, D. Heydenhauss, F. Hofmann, G. Jaenecke and L. Meister, Z. Chem., 16, 397 (1976); cited by Chem. Abstr., 86, 89743r (1977). 
PD3: S,S-Bis(4,6-dimethyl-2-pyrimidyl) dithiocarbonate. CAS registration number is [61887-68-1] Reference: Same as PD2. (See above) 
PD4: Bis(4,6-dimethyl-2-pyrimidyl) trithiocarbonate (see Example 1 for synthesis) 
The alkyl, cycloalkyl, aryl and aralkyl groups of the above formula I preferably have from 2 to about 15 carbon atoms and most preferably 2 to about 8. A preferred R3 comprises 2-pyrimidine.
Vulcanization accelerators appropriate for use in the invention include benzothiazole-based accelerators, particularly 2,2xe2x80x2-dithiobis(benzothiazole) (MBTS), mercaptobenzothiazoles, thiophosphoric acid derivatives, thiurams, dithiocarbamates, diphenylguanidine (DPG), di-o-tolyl guanidine, xanthates, sulfenamides and mixtures of one or more of these accelerators.
The amount of accelerating agent employed when the rubber in the vulcanizable composition is SBR is greater than about 0.6 phr, and when the composition comprises natural rubber, at least about 0.5 phr. The upper limit in either case is preferably about 10.0 phr and most preferably about 3.0 phr.
Particularly effective sulfur-vulcanizable rubber compositions in accordance with the present invention include a composition comprising styrene-butadiene rubber, one or more of the above preferred pyrimidine derivatives, PD1, PD2, PD3, or PD4, and a mixture of the accelerators N,N-diphenylguanidine, N-t-6Butyl-2-benzothiazolesulfenamide, 2,2xe2x80x2-dithiobis(benzothiazole) and, sometimes, 2,2xe2x80x2-dithiobis(4-methylbenzothiazole). We have found that with SBR, the amount of accelerator should be greater than about 0.6 phr of accelerator, and for natural rubber at least about 0.5 phr of accelerator. Natural rubber has more reactive allylic sites for crosslinking than SBR and generally requires less accelerator for efficient crosslinking.
It may be effective, in lieu of directly providing a pyrimidine derivative of formula I in the composition of the invention, to provide precursors of such derivatives that lead to the formation of such derivatives, in situ.
Conventional rubber additives may also be included in the sulfur-vulcanizable rubber composition in accordance with the present invention. Examples include reinforcing agents such as carbon black, silica, clay, whiting and other mineral fillers, processing oils, tackifiers, waxes, phenolic antioxidants, phenylenediamine antidegradants, antiozonants, pigments, e.g. titanium dioxide, resins, plasticizers, factices, and vulcanization activators, such as stearic acid and zinc oxide. These conventional rubber additives may be added in amounts known to the person skilled in the art of rubber compounding. The reader is also referred to the examples that are described below.
For further details on these typical rubber additives and vulcanization inhibitors, see W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.
Finally, in specific applications it may also be desirable to include steel-cord adhesion promoters such as cobalt salts and dithiosulfates in conventional, known quantities.
The sulfur vulcanization process of the present invention can be carried out using means and equipment that are well known to a person skilled in the art. Suitable vulcanization procedures are described in W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.
A typical method comprises preparing a masterbatch consisting of rubber, carbon black, a vulcanization activator, and a processing oil, in an internal mixer such as a Banbury mixer or a Werner and Pfleiderer mixer, and subsequently adding a vulcanization system comprising sulfur and a vulcanization accelerator, and the hardness stabilizing pyrimidine derivative in accordance with the present invention to the masterbatch either in a low temperature mixer or on a two-roll mill, i.e. the productive stage of mixing. The uncured rubber composition is then vulcanized by heating, e.g., in a compression mold.
The invention vulcanization process typically is carried out at a temperature of 110-200, preferably 120-190, more preferably 140-180xc2x0 C. for a period of time of up to 12, preferably up to 6, more preferably up to 3 hours.
The composition of the present invention is useful in the manufacture of many articles, including pneumatic tires, e.g., for passenger cars and trucks, and industrial rubber goods, which comprise the rubber vulcanizate obtained by the method of the invention. The invention is illustrated by the following examples.